2 go 



NATURE 



[January 2^, 1895 



found to contain a survey of Echinoderm morphology 

 more extensive than that accorded to the morphology of 

 other groups by the same author, as well as some useful 

 original diagrams, such as that of Pentremites, on p. 96S. 



E. R.\Y Lankester. 



THE STUDY OF ROCKS. 

 Lehrbuch der Pitrographie. \'on Dr. Ferdinand Zirkel, 

 Ord. Professor der Mmeralogie und Geognosie an der 

 Universilat Leipzig. Zvveite giinzlich neu verfasste 

 Auflage. Three vols. Svo. Pp.2619. (Leipzig : Engel- 

 mann, 1S93-94..) 



THE appearance of a second edition of Prof Zirkel's 

 admirable " Lehrbuch der Petrographie " is an 

 event of no little importance in the history of geological 

 science The part played by the author of this work in 

 developing the methods of microscopic analysis, as ap- 

 plied to rocks, is too well known to require recapitulation 

 in this place, and the long series of petrographical 

 memoirs with which he has enriched geological 

 literature^dealing with the rocks of the -N'orth .-Vmerican 

 as well as with those of the European continent^are 

 familiar to all students. While a great number of works 

 treating of the microscopic study of rocks (including the 

 author's own "Die mikroscopische Beschaffen heit der 

 Mineralien und Gesteine ") have appeared during the 

 twenty-eight years which have elapsed since the first 

 edition of the "Petrographie" was published, there is 

 not one among them that qu ite occupies the place of that 

 excellent treatise — with its wealth of information on the 

 history anddevelop.nent of petrographical nomenclature. 



The rapidity of growth of this branch of geological 

 science daring the past thirty years, is brought out in a 

 very striking manner by a comparison of the first and 

 second editions of this standard work. The first edition 

 consisted of two thin volumes with an aggregate of 1241 

 pages; the second edition forms three bulky volumes 

 with 2319 pages. But this is not all : it is evident to 

 anyone who peruses these volumes, that, in spite of the 

 employment of more than double the number of enlarged 

 pages, with much small type introduced, the author has 

 found it impassible to discuss in all their aspects the 

 views of previous authors with the completeness and 

 comprehensiveness that were so remarkable in the first 

 edition of the book. The student may be satisfied that 

 Prof. Zirkel has overlooked little or nothing of importance 

 in the literature of his subject ; but, not unfrequently, it 

 will be noticed that in his attempt to deal as concisely 

 as possible with this vast mass of literature, he has made 

 statements with respect to the views of the authors quoted, 

 which are scarcely borne out by a reference to the 

 memoirs themselves. While, therefore, this new edition 

 will be invaluable in supplying ample references to petro- 

 graphical literature, it will not in any way obviate the 

 necessity of consulting the original memoirs. 



As indicating the fulness with which the subject is now 

 treated, we may mention that the number of pages 

 dealing with "General Petrography" has increased from 

 171 to 634. The account given of the optical properties 

 of minerals and of the structure of rocks, as made out 

 by the aid of the microscope, is naturally responsible for 

 a large part of this increase ; the description of the 

 NO. 1317. VOL. 51] 



common rock-forming minerals, which was comprised in 

 forty pages in the first edition, now requiring no less 

 than 291 pages. The greatest defect in this part of the 

 work will be found in the absence of illustrations. Of this 

 the author is fully sensible, as will be seen from a 

 reference to his preface ; but, as he justly pleads, the 

 addition of illustrations could not fail to add to the 

 bulk and cost of a book that has already grown to 

 encyclopasdic dimensions. 



Every student of geology will naturally examine the 

 work with the desire to learn what are the present views 

 of so great an authority as Prof. Zirkel on the vexed 

 subject of rock-classification and nomenclature. In the 

 first edition of the book, our author, following the plan 

 of most German writers upon the subject, attempted to 

 class rocks according to their structure and mineralogical 

 constitution, quite irrespectively of their origin, into 

 simple crystalline rocks (ice, rock-salt, quartz-rock, lime- 

 stone, S:c.), compound crystalline rocks of granular and 

 schistose character respectively, and clastic, or frag- 

 mental, rocks. In the second edition, he departs from 

 this method, and commences the descriptive portion of 

 the book with an account of the " Massige eruptive 

 Erstarrungsgesteine," which occupies no less than 1293 

 pages. This is followed by an account of the " Krystal- 

 linische Schiefer" (275 pp.), the " Krystallinische oder 

 nicht-klastiche Sedimentgesteine " (230 pp.), and the 

 " Klastichen Gesteine " (125 pp.). It will thus be seen 

 that the primitive classification into crysialline (simple 

 and compound) rocks and clastic rocks, has been 

 abandoned for one in which account is taken of their 

 mode of origin. 



In dealing with the great class of igneous rocks, Prof. 

 Zirkel has also introduced some modifications of his 

 original method. In 1 866, he grouped these rocks, ac- 

 cording to the nature of the alumino-alkaline silicate 

 present in them, into Orlhoclase-rocks, Oligo'clase- 

 rocks, Nepheline- and Leucite-rocks, Labradoriterocks, 

 Anorthite-rocks, and rocks without felspathic con- 

 stituents. The obvious objections to this classification 

 were: (i) That many rocks contain several distinct 

 species of felspar, notably in their pnrphyritic con- 

 stituents, and in their groundmass respectively ; and (2) 

 that geologists possess no simple, infallible, and easily 

 applied test for ascertaining the exact species of felspars 

 present in rocks. In 1873 (in his " Mikroskopisch 

 Heschaffenheit der Mineralien und Gesteine"), Prol 

 Zirkel abandoned this classification for the simple] 

 division of felspar-bearing rocks into Orthoclastic an 

 Plagioclastic. This classification, which is facilitated bj 

 the general presence of twin slrialion as an easily reco; 

 nised distinction of the plagioclases, has now been ve 

 generally adopted by petrographcrs. In the work befoi 

 us, however, the author divides the felspar-bearin| 

 igneous rocks into two series, distinguished by the pn 

 dominance of an "alkali felspar ' or of a " soda-li 

 felspar " respectively. It does not appear to us that an 

 thing is gained by this new departure, which will co- 

 pcnsate for the admitted difficulty of applying the tel 

 for the discrimination of the two classes. AnotW 

 change in classification which will interest Engli 

 readers is that, while the separation of volcanic rocR? 

 into two series, the pre-teitiary (pa'ao-volcanic) an 



